Most digital circuits are synchronous in nature, meaning that a clock signal controls data flow through the circuit. As clock speeds increase, circuit design becomes more complex due to the timing requirements. Problems related to high clock speeds include switching noise, peak currents on power rails, and unnecessary power consumption due to the switching noise. As a result of the problems encountered with synchronous circuit design, asynchronous design techniques have received more attention.
One such asynchronous approach is null convention logic (NCL). NCL is a clock-free delay-insensitive logic design methodology for digital systems. NCL uses a combination of multi-wire data representation and a control/signaling protocol. NCL circuits switch between a data representation of DATA and a control representation of NULL. Typically, DATA corresponds to a logic-1 level, while NULL corresponds to a logic-0 level. The separation between data and control representations provides self-synchronization, without the use of a clock signal.
The use of asynchronous circuit designs, such as NCL, may be advantageous in space, weapons, and aviation applications. However, these applications expose circuits to naturally occurring radiation. Radiation may be found in the form of alpha and energetic particles, as well as in other forms, such as gamma rays. Alpha particles are the byproducts of the natural decay of elements. Energetic particles include heavy ions, protons, neutrons, and electrons, which are abundant in space, even at commercial flight altitudes.
Radiation can cause transient disturbances, or glitches, in asynchronous circuit designs. When an energetic particle strikes a transistor region, a parasitic conduction path can be created, which may cause a false transition on a node. The false transition, or glitch, can propagate through the circuit and may ultimately result in the disturbance of a node containing state information, such as an output of a latch, register, or gate. The disturbance of a state node is commonly referred to as a single even upset (SEU).
The circuit implementation of the basic NCL building block gate uses a latch element that is sensitive to upset due to transient disturbances caused by radiation. Therefore, it would be beneficial to harden an NCL circuit from the effects of SEU. As a result of hardening, NCL circuits may be used in applications in which radiation is present.